A brain-computer interface (BCI), sometimes called a direct neural interface or a brain-machine interface, provides a direct communication pathway between a human or animal brain (or brain cell culture) and an external device. With advances in CMOS technology, there has been a significant progress in implementing multichannel implantable neural systems which will potentially enable the diagnosis of disease and establishment of a direct interface between brain and external electronic devices. However, chronic monitoring of the brain activities, such as neural spikes, EEG, ECoG, etc., is still a challenge, especially, in wireless ambulatory systems due to constraints in power, noise and area for hardware implementation.
Typically, field potentials (in EEG, ECoG) contain most of their useful information in the bandwidth between 0.5 Hz and 300 Hz with amplitude ranges from 1 μV to 100 μV, where thermal and 1/f noise can severely interfere with the signal. To establish the reliable monitoring of vulnerable neural signals, a front-end preamplifier is typically used and is one of the more significant components in determining the signal-to-noise ratio of the entire system. Preamplifiers should meet the requirements of low noise (<100 nV/√Hz) for reliable measurement of weak signals, low-power consumption (<2 μW) for chronic operation, and small area (<0.1 mm2) for multichannel systems. Furthermore, the interference at the electrode interface induces a DC offset and drift which can be easily up to 1˜2V and changes over time at very slow frequency (<0.1 Hz). In at least most cases, this offset must be suppressed to prevent the saturation of the amplifier.
Previously, many preamplifier circuit techniques have been reported in the literature. In order to achieve high noise-power efficiency, most amplifiers utilize closed-loop topologies with input transistors operated in subthreshold region where the gm/Id can be maximized. However, these closed-loop amplifiers impose the stability constraint which results in the limit to the power-noise efficiency. A low-power open-loop amplifier has been proposed by J. Holleman and B. Otis, “A Sub-Microwatt Low-Noise Amplifier for Neural Recording,” in Engineering in Medicine and Biology Society, 2007. EMBS 2007. pp. 3930-3933. However, the single-ended output is susceptible to common mode noise and supply fluctuation.
To suppress the electrode interface interferences, a simple passive high-pass filter (HPF) is widely utilized using a large input capacitor (10˜20 pF) which occupies most areas in the preamplifier. To address this, several other low-frequency suppression techniques have been introduced. However, the low-frequency corners of these amplifiers are not controllable. As reported in J. Parthasarathy, A. G. Erdman, A. D. Redish, and B. Ziaie, “An Integrated CMOS Bio-potential Amplifier with a Feed-Forward DC Cancellation Topology,” in Engineering in Medicine and Biology Society, 2006. EMBS '06. 28th Annual International Conference of the IEEE, 2006, pp. 2974-2977, a feed-forward method can be implemented in a small area by using a high-pass filter (HPF) with a low cut-off frequency (˜0.5 Hz). However, the cut-off frequency can be difficult to control and vulnerable to process variations.
Thus, there remains a need for an area efficient bioamplifier that exhibits low noise with a good linear gain that is substantially independent of current supply variations.